Worm Breeder's Gazette 8(2): 15

These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.

More on Cytoplasmic Determinants

E. Schierenberg, W.B. Wood

Using the laser to perform microsurgery on embryos as described in 
the previous newsletter we have asked whether fusion of a cell from 
one lineage with cytoplasm from a second lineage can lead to 
expression of a differentiation marker specific to the second lineage. 
Our preliminary experiments indicate that this can occur for at least 
one and possibly three different markers.
In the first set of experiments, a non-gut precursor cell (an AB 
descendant) was fused to cytoplasm from a gut precursor cell (P1).  
The posterior egg shell of a 2-cell embryo was punctured and the P1 
nucleus extruded with a small amount of cytoplasm.  The extruded P1 
fragment was separated from the embryo, and the P1 cytoplast (
enucleated cell) remaining inside the shell was fused to a descendant 
of AB.  After development overnight (to a terminal phenotype of 
several hundred cells without morphogenesis), more than 50% of the 
embryos showed the localized autofluorescence characteristic of 
differentiated gut cells in a normal embryo.  In control experiments 
with no fusion following P1 enucleation, no autofluorescence was 
usually observed.  Some embryos in both types of experiments showed 
diffuse low-level fluorescence perhaps resulting from degeneration.
In a second experiment, difficult to perform and successfully 
accomplished only once, the P1 nucleus was removed from one side of a 
1-cell embryo (to avoid possible loss of posterior determinants) just 
prior to completion of first cleavage, leading to reformaton of a 1-
cell embryo with an AB nucleus.  This embryo also developed to several 
hundred cells including differentiated cells of typical body-muscle 
appearance, and showed twitching indicative of muscle contraction.  
Twitching was never observed in control embryos such as those 
described above.  (This experimental embryo did not show gut 
autofluorescence.  Embryos in the first set of experiments were not 
observed to twitch, but only their terminal phenotypes were examined, 
so that twitching during development would have been missed.)
In a third set of experiments the AB nucleus was removed from the 
anterior pole during the first cleavage, leading to formation of a 1-
cell embryo with a P1 nucleus.  These embryos also developed to 
several hundred cells, often including a group of small cells at the 
anterior end whose nuclei appeared typical of neuronal nuclei in the 
Nomarski microscope.  A better marker is needed to show that these are 
neuronal cells.  Control experiments in which AB was enucleated after 
first cleavage showed no such cells.  We conclude that cytoplasmic 
factors can alter the potential of early nuclei for lineage-specific 
patterns of expression.